A large number of strapping devices for applying a band around goods to be packaged are known. Although these devices to some extent differ from one another sharply in design terms, they share the common characteristic that they each have a band moving device with which the band is pulled from a delivery spool and the feeding motion of the band for forming a band loop around the package is produced. After the band loop has been formed, it is necessary for the band to be applied tautly to the package. In order to produce the necessary band tension, the band is pulled back until the necessary band tension is applied. For this operation, the band moving device is also normally used.
In the known band moving devices, a distinction may in principle be drawn between two types. In the case of the first type, the band is guided over a circulating belt of a belt drive or envelope drive. In this case, the band is carried along by frictional forces between the belt and the band. In the case of the second type of band moving device, a feeding motion of the band is achieved by the latter being guided directly over one or more rollers. Here, too, a frictional force--namely between the rollers and the band--is used.
The present invention relates to the first type of band moving devices, which is disclosed, for example, by Swiss Patent 662 791. There, a description is given of a strapping device having a band moving device in which a belt is guided over a guide roller and a drive roller. In order that a frictional force that is necessary for the feeding and the tensioning of the band occurs between the band and the belt, an eccentric disc presses the band against the belt. In the case of this device, it has been shown to be particularly advantageous that the band does not rest on the guide roller only in the region of the wrap angle of the belt. Because of the eccentric disc, on the one hand a greater wrap angle of the belt around the guide roller is established. In addition, the effect of the eccentric disc is also that the band rests against the belt outside the region of the wrap angle, which results in a relatively large contact surface between the belt and the band. This type of band moving device is therefore predestined for the application of particularly high tensioning forces.
In spite of these advantages, the arrangement is not completely satisfactory, since band deformations may result, in particular because of the deflection of the band by the eccentric disc. These band deformations are particularly disadvantageous in the automatic hooping of packages using bands, since they make the satisfactory guidance of the band during the formation of the band loop more difficult, even if they do not in fact prevent it. Band deformations of this type can lead to automated strapping devices coming to a standstill. In order to restart the machine following standstill, and to ensure satisfactory operation, the deformed band section is as a rule removed as waste.